Method of preparing boron trialkyl



United States Patent 'fiice 2,891,997 Patented, June, 23, 1 959 METHODOF PREPARING BORON '13 Claims. (Cl. 260-6065) This invention relates toa method for preparing boron trialkyl.

Boron trialkyl readily reacts with hydrogen in an electrodelessdischarge tube to produce alkylated diboranes such as tetra-alkyldiborane, as disclosed in assignees copending application Serial No.470,240, filed November 22, 1954. Boron trialkyl also reacts withdiborane to form tetra-alkyl diborane. When added to hydrocarbon fuelssuch as gasoline in amounts corresponding to about 1% by weight, thenormally liquid tetra-alkyl diboranes have been found to dramaticallyimprove the combustion characteristics of such fuels in internalcombustion engines. Due to the difiiculty of economically obtainingadequate quantities of boron trialkyl, the commercial use of tetra-alkyldiborane has been seriously limited.

Heretofore, boron trialkyl has been prepared by reacting boron esters orhalides with a metal alkyl or Grignard reagent in ether solutions. Thesynthesis employing the Grignard reagent is not adaptable to flowtechniques and is therefore commercially impractical. The synthesisemploying metal alkyls is limited by the expense and generally hazardousnature of the metal alkyls. Boron trialkyl has also been prepared bypassing boron halide and an alkyl halide over a metal at hightemperatures. However, this method produces boron trialkyl in pooryield.

I have now found that boron trialkyl is readily obtained by reactingmagnesium boride with an alkyl halide. The reaction can be conductedeither in a static system or a flow system and is readily adapted tolarge scale commercial production. It employs neither expensive norhazardous starting materials and the products are readily recovered.

The reaction is generally conducted by heating the metal boride in anatmosphere of the alkyl halide. The reaction proceeds smoothly andrapidly at temperatures between about 225 C. and about 425 C. As amatter of commercial availability and convenience, alkyl chlorides areordinarily employed.

The following examples are presented to more fully illustrate thisinvention. It is to be understood, however, that these examples arepresented merely as a means of illustration and are not intended tolimit the scope of the invention in any way.

EXAMPLE I Preparation of trimethylboron by means of a flow techniqueMethyl chloride was passed over magnesium boride (20 to 42 mesh) heatedto 420 C. at the rate of 19.2 cc./min. (70 F. and 1 atm.) calculatedresidence time, 13.3 sec. A 44% conversion of the methyl chloride wasachieved with an 8.9% yield of trimethylboron. A duplicate run using aflow rate of 27 cc./min. (residence time 9.5 sec.) yielded 11%trimethylboron with a conversion of the methyl chloride, at 420 C.

EXAMPLE 11 Preparation of trimethylboron inv a-staticrsystem.

Magnesium boride was heated in an atmosphere. of methyl chloride(initial pressure=l27 mm. at 26 C.)

at a temperature of 412-418 C. for approximately onehalf hour. Thenon-condensable productsxfound' in the reaction were pumped ofi at 196C.. (liquid nitrogen). An infrared analysis of the .condensable fractionindicated a 74% conversion of the methyl chloride and a 16% yield oftrimethylboron based upon the conversion value.

EXAMPLE III Preparation of trimethylboron in a static system Magnesiumboride was heated in an atmosphere of methyl iodide at a temperature of385-405 C. for approximately 1 /2 hours. The methyl iodide underwent a78% conversion producing trimethylboron in 15% yield.

In accordance with the invention, methyl bromide reacts with magnesiumboride to produce boron trimethyl and propyl chloride reacts withmagnesium boride to produce boron tripropyl.

Any lower alkyl halide can be used in the practice of this invention.These include methyl, ethyl, propyl, butyl, etc., chlorides, bromidesand iodides. Mixtures of these compounds can also be used so as toproduce mixed compounds such as boron dimethylethyl, boronmethylethylpropyl, etc.

The preferred temperature range is from about 225 C. to about 425 C.Lower as well as higher temperatures can be used, however, the reactionproceeds very slowly at lower temperatures while at higher temperaturesdegradation and the formation of undesirable products is likely tooccur.

Residence time or the duration of the reaction can be varied over wideranges. The reaction of course proceeds farther toward completion withlonger times, hence it is not desirable to use extremely short residencetimes. At the same time, it is usually not desirable to use long periodsof residence time since little is gained and side reactions may occur.

The novel reaction of this invention lends itself to a continuousprocess. For example, methyl chloride can be continuously passed overmagnesium boride, the reactant gases continuously stripped of borontrimethyl product and the unreacted methylchloride recycled repeatedly.Also, methyl iodide can be continuously passed over magnesium boride,the reactant gases continuously stripped of boron trimethyl product andthe unreacted methyl iodide recycled repeatedly.

I claim:

1. A method of preparing boron trialkyl which comprises reactingmagnesium boride with lower alkyl halide.

2. The method of claim 1 wherein said lower alkyl halide is an alkylchloride.

3. The method of claim 1 wherein said lower alkyl halide is an alkylbromide.

4. The method of claim 1 wherein said lower alkyl halide is an alkyliodide.

5. The method of claim 1 wherein the reaction is conducted at atemperature in the range of from about 225 C. to about 425 C.

6. A method of preparing boron trimethyl which comprises reacting methylchloride with magnesium boride at a temperature in the range of fromabout 225 C. to about 425 C.

7. A method of preparing boron trimethyl which comprises reacting methyliodide with magnesium boride at a temperature in the range of from about225 C. to about 425 C.

8. A continuous method for producing boron trialkyl which comprisescontinuously passing lower alkyl halide over magnesium boride heated toa temperature of from about 225 C. to about 425 C.

9. The method ofsclaim 8 wherein the reaction prod uct is stripped ofboron trialkyl and the unreacted alkyl halide recycled and again passedover said magnesium boride.

10. A continuous method for producing boron trimethyl which comprisescontinuously passing methyl chloride over magnesium boride heated to atemperature of about 420 C.

11. The method of claim 10 wherein the reaction product is stripped ofboron trimethyl and the unreacted 4 methyl chloride recycled and againpassed over said magnesium boride.

12. A continuous method for producing boron trimethyl which comprisescontinuously passing methyl iodide over magnesium boride heated to atemperature of from about 385 C. to about 405 C.

13. The method of claim 12 wherein the reaction product is stripped ofboron trimethyl and the unreacted methyl iodide recycled and againpassed over said mag- 10 nesium boride.

No references cited.

1. A METHOD OF PREPARING BORON TRIALKYL WHICH COMPRISES REACTINGMANGNESIUM BORIDE WITH LOWER ALKYL HALIDE.